1. Field of the Invention:
This invention relates generally to an etching processing apparatus which processes a lithographic master used in an offset printing machine, and especially to an apparatus making a surface of the lithographic master hydrophilic.
2. Description of the Prior Art:
Generally, a surface of the lithographic master for an offset printing machine comprises an imaged part being receptive to an ink and a non-imaged part, or blank which is ink-repellent, or receptive to water. Accordingly, it is necessary to perform a prior processing in order to make the non-imaged part of the lithographic master hydrophilic, in case that the lithographic master bearing an imaged part, visualized by a developer provided in a plate making apparatus, is set to an offset printing machine. This process in which the non-imaged part of the lithographic master is made to be hydrophilic is called an "etching process". If the etching process is imperfect, the ink adheres to the non-imaged part of the lithographic master during printing, so that any good image printing cannot be obtained and an enduring printing ability of the lithographic master to transfer the imaged part of a surface of the lithographic master to a plain paper decreases. Accordingly, a process for making a surface of the lithographic master hydrophilic is very important in offset printing.
Meanwhile, a conventional etching process generally employs the following two methods, though the etching process depends substantially on the material of the lithographic master. In the first etching method, the surface of the lithographic master is rubbed by a sponge, or an absorbent cotton, including an etching liquid, so as to form a thin hydrophilic film on the non-imaged part of the lithographic master, but this method is performed by manual work so that the quality of the etching process depends upon the experience of the worker, and further it takes a long time to perform the etching process. As a result thereof, this etching process is at a disadvantage where the etching is performed for making multiple lithographic masters.
According to the second etching method, the lithographic master itself is soaked in an etching vessel. However, this second method is superior to the first etching process method in the respect that an etching process for multi-lithographic masters can be performed and a uniform etching effect can be obtained, but in this second method, only the lithographic master soaks. As a result thereof, the second method has the following defects. First a pinhole-state blemish is produced due to a dust or bubble adhering to the surface of the master paper, so that the hydrophilic film cannot be formed at the position of the pinhole. As a result, there happens that the blemish of the master paper produces on a printing paper. The above-mentioned pinhole can be eliminated by soaking the master paper in the etching liquid vessel. However, the mechanical intensity of the lithographic master itself tends to fail when the lithographic master is soaked for a long time, where the lithographic master, including zinc oxide and the like, is used in the offset printing. Furthermore, to soak the lithographic master in the etching liquid vessel for a long time causes lowered efficiency of the etching process.